Why Are My Watermelons Turning Black?

Most of a watermelon’s heavy weight is due to its high water content.

Watermelons (Citrullus lanatus) do best with a long, warm summer, although you can grow them in most areas. While they’re relatively easy to grow, watermelons are vulnerable to some diseases and cultural problems. Some of these issues can cause the fruit to turn black.

Anthracnose Fungus

Anthracnose fungus (Colletotrichum orbiculare) infects watermelon leaves, stems and fruits. It develops into angular or irregular, dark brown to black lesions on the leaves and stems at first, but if the infection is prolonged, the fruit develops raised lesions with sunken centers, which can be black. To prevent the spread of anthracnose to other plants, don’t work in your garden when the vines are wet and remove all infected fruit and plant debris. Severe infections can be treated with fungicides containing mancozeb as the active ingredient. Cover the fruit to avoid contact with the fungicide, and spray at a rate of 4 to 5 teaspoons of fungicide per 1 gallon of water. Make sure you cover both upper and lower leaf surfaces, and do not apply within five days of harvest. Repeat applications at seven- to 14-day intervals, and wait five days after application before harvesting fruit. When applying any fungicide, read and follow all label recommendations regarding safety and application rates and timing.

Blossom End Rot

When a watermelon plant is deficient in calcium or suffering from drought or excessive nitrogen, it may develop blossom end rot, which shows as a pale green to brown to black discoloration on the end of the fruit where the flower was. The discoloration enlarges into sunken spots and the blossom end shrivels into a dry rot unless the tissue is invaded by secondary organisms, which can extend the rot throughout the fruit. Prevent blossom end rot by keeping the watermelons well watered and don’t over-fertilize with nitrogen or potassium fertilizers, which can reduce the uptake of calcium. Treat calcium deficiencies by applying a foliar spray solution of 4 tablespoons calcium chloride per 1 gallon of water or 5 tablespoons calcium nitrate per 1 gallon of water. Apply the spray in the morning and cover the leaves thoroughly, wetting the undersides as well as the leaf surfaces. Repeat at seven- to 10-day intervals, for a total of three or four applications. This will prevent new fruits from being damaged, but will not help those already showing signs of blossom end rot.

Black Rot

Black rot is caused by the fungus, Didymella bryoniae, which also causes gummy stem blight of the stems and leaves. Symptoms begin as small water-soaked lesions on the fruit that may exude gummy ooze. The lesions enlarge, becoming sunken and discolored, and black fruiting bodies appear. To control black rot, remove and destroy infected fruit and all plant debris. Rotate watermelon crops with non-host crops, such as annual corn (Zea mays), for two or more years after infection, and avoid overhead watering so the leaves stay dry. Severe black rot infection can be treated with fungicides containing mancozeb as the active ingredient. Cover the fruit to avoid contact with the fungicide, and mix 4 to 5 teaspoons of fungicide in 1 gallon of water in a garden sprayer. Spray all leaf surfaces, and wait at least five days before picking the watermelons. Repeat every one to two weeks.

Belly Rot

Belly rot is associated with the soil-borne fungi, Pythium aphanidermatum and Rhizoctonia solani. Symptoms begins as small, water-soaked areas on the bottom of the watermelon, where it comes in contact with the fungi through the soil. The fungus colonizes the fruit and causes it to rot. The affected area may also be surrounded by abundant growth of white mycelium. Belly rot is most severe during wet periods and high temperatures. You can control and prevent belly rot by staking or caging the fruit to avoid direct contact with the soil.